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The MTF & DQE of Annular Dark Field STEM: Implications for Low-dose Imaging and Compressed Sensing
摘要： Annular dark-field imaging in the scanning transmission electron microscope (ADF-STEM) is a hugely useful incoherent imaging mode, yielding monotonically increasing mass / thickness contrast facilitating quantitative compositional mapping, atom-counting, structure-solving and even unstained biological-imaging. With increasing interest in beam-sensitive materials, many people would like to perform ultra-low-dose imaging or compressed-sensing (CS). Unfortunately, some CS implementations require complex blankers/mask/shutters and may require expensive instrument modification; fast-scanning on the other hand should in principle be available to everyone. Nearly all installed ADF detectors follow the same basic design comprising; a scintillator, light-pipe, PMT, amplifier and a digitiser outputting in arbitrary uncalibrated units. While the scintillator material itself may have an afterglow of only some tens of nanoseconds, the combined assembly has a decay constant of the order of 1.7-3μs which becomes significant in some cases. Scanning with dwell-times faster than this decay introduces issues such as increased background and loss of contrast, streaking in real-space and a diffuse noise band in Fourier-space. In spite of these problems, previous literature has shown that PMT based detectors are in fact sensitive to even single electron signals, unfortunately they are also highly inhomogeneous in their collection efficiency. One solution to this is to as whether we can form an image using these signals using a pulse read-out? Such an image would record the arriving electron as a single impact-event rather than a streak (perfect MTF), the results would be digital rather than analogue, and all electrons would be recorded with equal sensitivity (perfect DQE). The integer nature of the resulting image signal would also directly benefit quantitative contrast (fractional-beam) studies, statistical image analysis, and compressed-sensing / image-inpainting. In this presentation, we will present a new tool to generate realistic noise realisations from ADF image-simulations. This tool registers each electron of a finite dose passing through the sample, to the detector (incorporating afterglow), and onward to amplifiers with realistic thermal and electronic noise. This tool is verified against previous experimental observations of the behaviour of fast-scanned images, and is used to explore the modulation transfer function (MTF) of ADF-STEM for various dwell-times. We then propose, and demonstrate using both simulations and experiment, a new method for operating an ADF detector in a pulse read-out mode (which we call ADFpro) using dwell-times of 0.5μs and 0.2μs. The detector’s quantum efficiency is evaluated indirectly through the evaluation of image pixel-value histograms but also directly using ADF-detector sensitivity maps. The limitations of this approach will be presented, as well as the implications for ultra-low-dose imaging and for compressed-sensing / image-inpainting using such fully digital data.
关键词： ADF-STEM,MTF,DQE,compressed sensing,low-dose imaging
更新于2025-09-10 09:29:36
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The feasibility of low-dose CT perfusion imaging in gastric cancer
摘要： PURPOSE: To investigate feasibility of applying low-dose CT perfusion imaging (CTPI) to diagnose gastric cancer. MATERIALS AND METHODS: Twenty patients with gastric cancer con?rmed by endoscopic biopsy were undergone routine dose (120 kV, 100 mA) and low-dose (120 kV, 50 mA) CTPI examination, respectively. The original data were processed by body perfusion software, and the perfusion parameters values including blood ?ow (BF), blood volume (BV) and permeability surface (PS) of gastric cancer were measured. Statistical data analyses including paired-samples t test, Pearson correlation analysis and Bland-Altman consistency test were used to compare the perfusion parameters values between the routine dose and low-dose CTPI examinations. Radiation dosage, which the patients received during two CTPI examinations, was also calculated and compared. RESULTS: There were no statistical differences in the BF, BV and PS values between routine dose group and low-dose group (P > 0.05), and there were signi?cant correlation in the BF, BV and PS values between two groups (P < 0.01). The consistency of BF and BV values between the two groups was preferable to that of PS value. The radiation dosage of the low-dose group was much less than that of routine dose group, and the CTDIvol and DLP values of low-dose CTPI were decreased by 50%, respectively. CONCLUSION: The parameters BF and BV values may play a valuable role in the diagnosis and assessment of gastric cancer in low-dose CTPI examination.
关键词： gastric cancer,computed tomography,Low-dose,perfusion imaging
更新于2025-09-10 09:29:36
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Re-evaluation of the correction factors for the GROVEX
摘要： The GROVEX (GROssVolumige EXtrapolationskammer, large-volume extrapolation chamber) is the primary standard for the dosimetry of low-dose-rate interstitial brachytherapy at the Physikalisch-Technische Bundesanstalt (PTB). In the course of setup modifications and re-measuring of several dimensions, the correction factors have been re-evaluated in this work. The correction factors for scatter and attenuation have been recalculated using the Monte Carlo software package EGSnrc, and a new expression has been found for the divergence correction. The obtained results decrease the measured reference air kerma rate by approximately 0.9% for the representative example of a seed of type Bebig I25.S16C. This lies within the expanded uncertainty (k = 2).
关键词： dosimetry,ionisation chamber,extrapolation chamber,low-dose rate brachytherapy,primary standard
更新于2025-09-09 09:28:46
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Evaluation of a novel photon-counting CT system using a 16-channel MPPC array for multicolor 3-D imaging
摘要： X-ray computed tomography (CT) has been widely used in the diagnostic imaging of the interior of the human body. However, the radiation dose of conventional CT typically amounts to 10 mSv. Under such environments, X-ray photons are severely piled-up; therefore, conventional CT acquires energy integrated images, and artifacts are formed by beam hardening. In contrast, a photon counting CT (PC-CT) system is anticipated to construct a low-dose and multi-color CT system. Recently, we proposed a novel PC-CT system using a multipixel photon counter (MPPC) coupled with a high-speed scintillator, which is cost effective and easy to assemble compared to other methods using CdZnTe device. In this paper, we report the results using an advanced CT system consisting of a 16-channel MPPC and scintillator array coupled with a newly developed large-scale integrated circuit (LSI) having an ultrafast signal processing capability. We present the performance of the photon-counting CT capability, such as the contrast of the obtained CT images compared with that of the current-mode CT, and we found that substantial reduction in radiation dose by an order of magnitude. In addition, we report the results of three-dimensional multicolor imaging to identify phantom materials.
关键词： low-dose,MPPC,Photon counting CT,Multicolor image
更新于2025-09-09 09:28:46